DIY Compound Bow Scope for Any Light Condition — 3D Printed and Illuminated | SparrowDot V3"

"Light conditions change constantly — and you need to be able to adapt your scope to match them. Indoor or outdoor. Dawn, noon, dusk or night. Every situation demands optimal visibility. That's what drove me to build SparrowDot. That — and the challenge. 😊

Spanning 3 years, three versions, one very small LED — SparrowDot was born. Precision 3D printing, electronics, and basic optics — something no commercial scope currently offers. Open source and free.

Build one for yourself or for your local archery community. Want to connect with archers near you? Visit sparrow-dot.com

Who knows — you might end up an archer yourself.

The image above gives you a preview of the full Bill of Materials — click the link below to open the interactive version with live sourcing links.

All parts are standard and easy to source on AliExpress or similar suppliers. The BOM is updated from time to time as we find better sources or more reliable suppliers.

Printed parts are available as free downloadable files on our website and on Printables. For the most current versions and updated files, always check sparrow-dot.com first.

Prefer to skip the printing? We may have a solution for you — get in touch through sparrow-dot.com"

A note on components and hardware: some values and parts in this design were chosen around what I had in stock at the time — both electronic components and mechanical hardware such as screws, rods and fasteners. If you're an experienced maker and have suitable equivalents on hand — go ahead and substitute. Calculate electronic values carefully before soldering, and verify mechanical fit before gluing or embedding. The design is forgiving of sensible substitutions on both fronts.

Open the full interactive BOM here:

Step 1 — Understanding the Project

As you can see from the product tree, SparrowDot V3 consists of four main assemblies. The Reticle and the Eyepiece Ring are built as separate, interchangeable assemblies. The other two — the Main Body and the LED Control Circuit — are each assembled independently, then combined to form the complete scope body.

A fully functional scope consists of the main body, a lens, a reticle, a level bubble, and an eyepiece ring — all chosen by the archer and easily swapped in the field. Each element is interchangeable, allowing the scope to adapt to any shooting condition.

Additional accessories — sunshade, the V3 Nest, extra lenses and more — may be added to complete the SparrowDot V3 Chameleon Edition. A fully equipped system, ready for any condition, any competition, any range.

Preparing Your Printed Parts

Before any assembly begins — take the time to prepare all your printed parts correctly. This step is often skipped and always regretted.

General preparation — all parts:

1. Clean and lightly sand all parts — remove any print artifacts, blobs or rough edges
2. Keep all surfaces matte — inside and out. No polishing, no shine. A shiny surface inside the scope body causes internal reflections that affect visibility. A shiny exterior catches light and creates glare — exactly what we're trying to avoid
3. Test fit every part before gluing or assembling anything

Main body:

Print options:

1. Matte black — blocks light, eliminates glare, best for bright conditions
2. Translucent — glows passively in low light, best for dusk and indoor ranges
3. Combined — print the front section (lens to eyepiece) in translucent and the rear section (lens to back) in matte black. Best of both worlds
4. The level bubble groove must be tight — the bubble housing should require light force to seat fully and stay firmly in place without glue
5. Clean the lens seat carefully — any debris here will show in your sight picture
6. Verify the eyepiece ring connectors slide smoothly along the full travel and lock firmly in the last millimeter — this is the locking mechanism, it must engage positively
7. 
   

Reticle frame: Matte black

1. Clean and shape the four cones carefully — they guide the LED leads and silicone wires
2. Free all six 1mm holes using a hand drill — they must be clean and clear before assembly begins
3. A blocked hole discovered mid-assembly is a serious problem

Eyepiece ring:

Print options:

1. Translucent — works with all three COB colors (green, orange and pink). The ring glows in the COB color when illuminated, and appears clear when the COB is off
2. Colored (green, orange or pink) — match or contrast with your reticle color. When illumination is off the ring color serves as a passive visual reference — useful for competitions where illumination is not permitted
3. Matte black — the traditional scope look. Clean and neutral, no color reference

1. Clean and lightly sand the sliding cylindrical male connectors — they must slide smoothly along the full travel with light friction
2. At the last millimeter the connector should require some force to lock firmly into place — this is intentional, it is the locking mechanism
3. Keep the inner surface matte — no polishing, no shine
4. Check the COB LED channel is clean and free of any debris before assembly

Take your time here — a well-prepared print makes every assembly step easier and the final result significantly better.

Recommended Assembly Sequence

We recommend working from the most demanding assembly to the easiest — this way your most critical work happens when your hands are fresh and your focus is at its best.

Step 2 — LED Control Circuit

Straightforward for anyone with perfboard soldering experience — but working space is tight. Take your time with the dedicated connector soldering and make sure the micro switch is soldered to float slightly, not sit flush.

Step 3 — The Reticle

The most delicate assembly of the four. Gluing the silicone cross wires, positioning and gluing the 0201 LED and its micro wires — all under a magnifying glass. Soldering the contacts and carefully placing everything into the printed frame.

Be at your best for this step. Work slowly. Think before every move. Be patient.

Every note and comment in this guide has a failure behind it — read them carefully.

Step 4 — Eyepiece Ring

The easier of the two illuminated assemblies — but precision still matters for the COB LED angle and connector soldering.

Step 5 — Main Body

Preparing the printed parts, adding the lens supports, inserting the metal connectors and battery contacts, then seating the LED Control Circuit inside and closing the body.

Once you complete Steps 2 and 3 you have passed the 20% of the work that takes 80% of the time. Everything after that feels like a reward.

Step 6 — Assemble the SparrowDot v Scope

Bring all four assemblies together into a complete, range-ready scope.

Step 7 — The V3 Nest — A Safe Home for Your SparrowDot V3

Three options for keeping the V3 and the Chameleon Edition safe. Two are 3D printable — purpose built for the scope. The third is a purchased standard case — not custom fitted but a practical alternative.

Now — let's dive into it. 🎯

Straightforward for anyone with perfboard soldering experience — but working space is tight. Take your time and follow the photos carefully.

Prepare the perfboard:

Cut it to size. Count the dots and refer to the photo above for the correct shape. Several cutting methods work depending on your tools — a Dremel or similar rotary tool gives the cleanest result (tool #1 in the photo).

1. Prepare the connecting wires — as shown in photos 3 and 4. Each grid square is 5mm × 5mm for reference.
2. Place and solder the variable resistors and LDR — follow photo 6 for placement. The LDR should sit approximately 2mm higher than the trimmers — this improves light sensing accuracy. For a more secure hold, add plastic sleeves of any kind on the LDR legs before soldering.
3. Solder on the top layer of the perfboard wherever possible — this keeps the bottom clean and makes fitting into the chamber easier. Additional photos and a short video of this process will be added.
4. Disassemble the 2032 battery housing — harvest the micro switch and install it in its designated position, soldered to float slightly above the perfboard — not flush.
5. Mark the + and − holes — these will be used later to connect power to the circuit. Mark them clearly now before the board goes into the body. Do not use the first row of holes for the power connection — leave them clear.
6. Test the board before final assembly — outside the chamber, connect the LIR2032 and both LEDs and verify everything works correctly. Fix any issues now — it's much harder once it's inside the body.

The reticle is the most delicate part of the entire build. To ensure first-time success, pay close attention to your tools and workspace. You need good light from the right angle — enough to illuminate the printed reticle without blur or shadows. Use a vice or clamp to hold the print steady, and keep a magnifying glass close for accurate work.

Read through the entire process before you start. Make sure you understand each step and know which photo relates to which section. Contact us if anything is unclear.

Bill of Materials:

Printed reticle frame, colored silicone wire (green, pink, orange — or black for the traditional option), 0201 LED, USB2 female connector, cyanoacrylate gel. Loctite Super Glue Ultragel is my personal favorite.

Special tools:

Magnifying glass, plastic toothpick, matches (not a lighter), sanding sponge, and a basket full of patience — rare on the market. 😊

It's easier to ruin than to build.

1. Prepare the printed part

1. Use a 1mm hand drill to clean all 6 holes around the printed ring
2. Clean and lightly polish the ring and the four cones
3. Position the ring with the front facing away from you — most of the work is done on the back side (the contacts plane faces away from you)

2. Insert the silicone ribbon — horizontal axis

1. Thread the ribbon through the horizontal axis holes
2. Make sure the wide side of the ribbon is vertical relative to the ring
3. Glue the far end with a small amount of cyanoacrylate gel — push it slightly back into the hole to ensure the gel penetrates. Let it cure
4. Pull the ribbon gently from the other side to create tension — glue that end while maintaining the tension. Let it cure

3. Insert the silicone ribbon — vertical axis

1. Thread and glue the vertical ribbon the same way
2. Keep it behind the horizontal ribbon when viewed from the front of the reticle

4. Thread the LED leads

1. Thread the LED leads through the holes observing correct polarity — LED positive on the right side when facing the reticle front
2. Front orientation: the contacts plane is facing toward you

5. Position the LED

1. Carefully position the 0201 LED at the center of the cross, on the vertical ribbon
2. Align the leads parallel to each other, behind the ribbon — do not attempt to wrap the two leads around each other
3. To hold it temporarily in place, bend the leads slightly at the top of the reticle

6. Glue the 0201 LED

1. Using a plastic toothpick, pick up a tiny dot of cyanoacrylate gel and place it at the center of the vertical ribbon — with the LED leads facing the ribbon (the LED emits light away from the ribbon)
2. When dry, position the leads parallel to each other as shown in the photo
3. First glue the top leads behind the cone — when dry, use the toothpick to apply small dots of glue along the leads on the silicone ribbon. Let everything cure for a few minutes

7. Prepare the leads for soldering

1. Straighten the leads — do not cut them yet. Cut only after soldering is complete and tested
2. The leads are coated with varnish — it must be removed without melting the copper. Hold the leads briefly at the tip of a match flame for no more than one second. Use a match — not a lighter. A lighter runs too hot
3. Lightly sand the burned residue with a ~600-grit sanding sponge
4. Apply a thin coat of solder to each lead (tin the leads) as close to the ring as possible without damaging it

8. Prepare and install the contacts

1. Disassemble a USB2 connector to extract the 4 contacts inside
2. Bend the narrow back section of the contact to vertical — cut to leave approximately 3mm. This will be inserted into the lead hole
3. Wrap the lead wire around the serrated section and solder it
4. Insert the 3mm bent section into the 1mm lead hole — hold it in position above the dedicated groove and sink it into the ring using your soldering iron at a lower temperature than used for the leads
5. Once soldering is complete and tested — carefully trim any excess lead wire close to the solder joint

9. Test the LED

1. Test the LED using a LIR2032 battery through a 470Ω resistor
2. Verify that polarity is correct and the LED lights up

10. Take a break — you deserve it! 😊

Compared to the reticle, the eyepiece ring is a walk in the park — but a few steps require precision due to tight dimensions. Take your time and it will reward you.

Before you start — choose your color strategy:

1. Matched colors — green COB in a green ring, orange in orange. When the COB is off the ring color remains as a passive visual reference — useful for competitions where illumination is not permitted
2. Translucent ring — any COB color with a clear/translucent housing. When the COB is off the ring appears clear. When lit, the full glow of the COB color shows through
3. The first photo shows a green COB in a translucent ring — off it looks clear, on it glows green

Match your eyepiece ring color to your reticle choice — or deliberately contrast them. Your scope, your call. Need more to know? go to sparrow-dot.com

Bill of Materials:

1. 3D printed eyepiece ring
2. Wire ferrules (cable end sleeves) x2
3. COB LED filament
4. Magnifying glass
5. Wooden toothpick — not plastic

1. Prepare the printed part

1. Clean and lightly sand the sliding cylindrical male connectors
2. The fit should slide smoothly with light friction — at the last millimeter some force should be needed to lock it firmly into place
3. This is intentional: the scope is subject to real forces in use, all parts must be firmly anchored — yet quick to swap in the field

2. Solder the wire ferrules

1. Apply a thin coat of solder to both ends of the COB filament — tin them first
2. The end with the hole is positive — it goes on the right side when working on it (when the COB is visible), consistent with the reticle orientation
3. Place a ferrule on the thin end of a wooden toothpick and fill the wide inner side of the ferrule with solder — repeat for both ferrules
4. Test before soldering: connect the COB to a LIR2032 battery through a 470Ω resistor — confirm it lights and verify polarity is correct
5. The COB filament is cut to the exact length needed — do not shorten it further
6. Solder the COB ends inside the ferrules — solder only the last 2mm
7. Insert the ferrules through the contact holes perpendicular to the print — a small amount of force is expected
8. Make sure the ferrules are pushed all the way in — apply cyanoacrylate gel at the back of each ferrule to fix it firmly in place. Hold until cured
9. Wrap the COB carefully into its place around the ring — try not to let it twist around itself.

3. Test it again

1. Reconnect the LIR2032 through the 470Ω resistor
2. Verify the COB lights correctly and the ferrule connections are solid

That's it — popcorn time. 🍿😊

The final assembly — the one that brings everything together to form the SparrowDot V3 scope. We add the power system, embed the connectors, and prepare the body to hold all parts firmly while keeping them quick and easy to swap.

1. Embedding the ferrules in the body

1. Place a ferrule on a straight scribe, wide end (the crown) facing down
2. Note: a metal scribe works but is not ideal — a firm non-heat conductive tool like a Spudger made from Teflon or nylon is more suitable and won't conduct heat
3. Apply a micro amount of cyanoacrylate glue at the print hole opening
4. Hold the ferrule close to or touching a hot soldering iron — when very hot, insert it into the hole
5. Stop when the wide crown is flush with the surface — do not embed it deeper
6. Repeat for all 4 holes

2. Bending 2 ferrules to ~90°

This is a two-step process — take your time:

1. First punch a small hole in the copper ferrule: insert the back of a 2mm drill bit until it aligns with the print surface. Using a sharp metal scribe just above the print surface, apply moderate force with micro movements to create a hole in the ferrule
2. Once done, insert the scribe into the ferrule and bend it slowly to approximately 80-90° — work slowly to maintain the round profile and prevent cracking

3. Place lens buffers — refer to photo 3 for placement.

1. Cut three pieces of TPU filament — 5mm length, 0.7mm diameter
2. Glue them with cyanoacrylate into the dedicated mounting groove just before the lens seat
3. They should fit snugly inside the groove — trim slightly if needed for a clean fit

4. Prepare the power system

1. Disassemble the 2032 battery housing to extract the two contacts and the micro switch
2. Install the switch as described in Step 2
3. Thread the positive and negative wires through the battery chamber and solder both into the perfboard as shown in Step 2
4. Insert the 0.8mm micro pin — rising from the printed body at the negative contact location — into the dedicated hole in the negative contact
5. Bend and place the positive contact as shown in the photo
6. Clean and make room as needed using a sharp utility knife at both contact locations
7. Questions about this step? Get in touch at sparrow-dot.com

5. Install the 10/32 mounting rod

1. Confirm that 10/32 is the correct thread for your bow sight mount before proceeding
2. Apply epoxy to glue the rod to the main body
3. Make sure the rod sits a few threads back from the nut to fit correctly into the dedicated hole in the main body
4. Choose the correct mounting hole for right or left handed shooter
5. Allow the full recommended epoxy drying time before continuing — do not rush this step

6. Insert all parts into the main body

1. Insert in this order: lens → reticle (your chosen color) → level bubble → eyepiece ring
2. On first assembly, manually clean and adjust each part to fit easily but firmly — use a utility knife or sanding sponge where needed for a smooth but tight fit

Hold on — we're almost done. One last push. 🎯

You made it. Now let's make sure everything works before you mount it on your bow.

1. Final checks
2. Insert the LIR2032 battery
3. Do the eyepiece ring and reticle glow? ✅
4. Using a small screwdriver adjust both trimmers — turn them back and forth. Does the brightness of each LED change independently? ✅
5. Cover the LDR (light sensor) with your finger — does the reticle dim automatically in response? ✅
6. Do all parts feel firmly connected to the main body — no wobble, no play? ✅
7. Do all interchangeable parts swap smoothly but lock firmly? ✅

If all answers are yes — SparrowDot V3 is ready for the range.

Mount it, zero it, and go shoot.

Share your build with us at sparrow-dot.com — we'd love to see it.

Everything your SparrowDot needs — in one safe, purpose-built solution.

The V3 Nest

Print in PLA or PETG depending on your climate — remember that PLA softens at 40°C (104°F). Never leave it in a hot car.

The complete Chameleon experience in one box. Holds the main scope body, 3 additional reticles and eyepiece rings (4 total including the one already assembled on the scope), 2 spare lenses, a spare LIR2032 battery, and accessories. All 3D printed by you.

V3 Travel Shell

Print in lightweight PLA (PLA LW) for minimal added weight.

A minimal 3D printed shell — the perfect protection for your SparrowDot V3 when you need to travel light without the full Chameleon kit.

V3 Travel Case

Prefer something ready-made? A standard purchased protective case is listed as an option in the BOM — not custom fitted but a practical alternative.

All files available as free downloads at sparrow-dot.com

Getting this far is no small achievement — it takes patience, steady hands, and real dedication. You genuinely deserve a round of applause. 👏

If your first build wasn't perfect — welcome to the club. Neither was ours. That's what versions are for.

Watch this space — the Sparrow ecosystem is growing. V4 is already on the drawing board — including a combined design with an integrated peep sight, or a hybrid approach. Whether you shoot it or build it — your ideas matter. Get in touch at sparrow-dot.com and help shape the next version.

Stay tuned. The best is still ahead.

— Ron

Archer & Maker — really working on things I love."

Connect with archers and makers in your area — visit sparrow-dot.com

— Ron

Archer & Maker — really working on things I love.